High integrity pressure protection systems (HIPPS) are safety instrumented systems built according to the International Electrotechnical Commission's International Standards IEC 61508 entitled “Functional safety of electrical/electronic/programmable electronic safety-related systems” and IEC 61511 entitled “Functional safety—Safety instrumented systems for the process industry sector” and designed to protect equipment downstream of the HIPPS, personnel and the environment from an overpressurization event. Subsea HIPPS are used in offshore oil and gas production to mitigate the risk of subsea flowline pressures exceeding tolerable pressure. The protection against excessive pressure is obtained by quickly isolating the source of a large increase in pressure. HIPPS include valves such as block valves capable of quickly shutting off flow through the HIPPS, pressure sensors for detecting the pressure of fluid flowing through the HIPPS, actuators (pneumatic or hydraulic) for controlling the valves, and logic controllers for receiving information from the pressure sensors and sending signals to the actuators. HIPPS typically include redundant components for reliable operation. The term “HIPPS” is used throughout to refer to high integrity pressure protection systems, either in the singular or the plural. HIPPS have been used on topsides oil and gas production facilities for many years.
In the past several years, the use of HIPPS in subsea environments has become a viable option for a number of reasons. For one, in some cases, subsea equipment cannot be manufactured to the required pressure rating. In some situations, subsea flowline or pipeline having the required pressure rating cannot be installed using conventional installation techniques due to the wall thickness that would be required to withstand the maximum pressures encountered for a given oilfield. In the case of a long distance tie-back, a HIPPS may enable use of optimized wall thickness pipe designed for lower pressure than the shut-in pressure, with a significant cost benefit and reduction of offshore installation risks given the pipe length required. In the case of a new high pressure tie-in to existing subsea facilities, a HIPPS may be needed because the existing subsea facilities are rated for a significantly lower pressure than that of the new well or field. A HIPPS may also be called for when the cost of the HIPPS is lower than the cost of deploying pipeline at the required pressure rating or when a production facility without the HIPPS would be otherwise uneconomic.
Shut-in tubing pressure (SITP) is the maximum pressure that occurs during non-flowing conditions in a flowline system when exposed to full source pressure. Currently, typical subsea flowline systems have SITPs of less than 15,000 psi and design temperatures of less than 350° F. The technology qualification process for subsea production systems rated up to 20,000 psi is particularly lengthy and involved, as is the lead time for delivery of subsea equipment. Since operating pressure is generally much lower than SITP, a flowline system rated for SITP is inherently overdesigned for intended operating conditions.
FIG. 1 illustrates a prior art offshore oil and gas production facility including a drill center 2 having one or more subsea wells, a fully rated section of pipeline 4 connecting the drill center 2 to a HIPPS 6, a fortified section of pipeline 8 connected to the HIPPS 6 followed by a de-rated section of pipeline and riser 12 that delivers produced fluids to the production facility 14. As shown in FIG. 1, immediately downstream of the HIPPS 6 is the fortified section, also referred to as a “fortified zone” 8, to protect the flowline and other equipment immediately downstream of the HIPPS 6 from rapid pressure increases which can occur as fluid passes through the HIPPS valves during the valve closure process. This is necessary since closure of the HIPPS valve is not instantaneous. The length of the fortified zone needed is based on the reaction time of the HIPPS valve when closing. This fortified zone 8 must be rated to full SITP in order to withstand the transient build-up pressure in case there is a blockage along the line. A fortified zone having a shorter length would be desirable since the fortified zone entails thick, heavy pipe walls, more complex fabrication and more onerous installation methods.
Despite the benefits offered by HIPPS technology, there remain drawbacks to HIPPS that have not successfully been addressed to date and are particularly prominent in oil-producing systems. For one thing, the faster the required closure time of the HIPPS valve, the more complex the HIPPS system needs to be in order to achieve rapid closure with the necessary extremely high degree of reliability. The HIPPS valve must close before the system pressure rises above the design pressure at the end of the subsea fortified zone 8. For another thing, the length of the fortified zone 8 that is required using known technology can often result in high cost and complex design, fabrication and installation requirements.
It would be desirable to have an improved method and system for preventing overpressurization of subsea equipment and flowlines, i.e., thereby lessening pressure increase, in a simpler, less costly way. It would further be desirable to have a technology that would allow the length of the fortified zone to be reduced, enabling the use of lower cost pipeline, as well as simpler welding, manufacturing and installation requirements. It would also be desirable to have a technology that would allow the required closure time of the HIPPS valve to be lengthened, thereby reducing cost and complexity of the HIPPS valve(s).